Osteoarthritis (OA), also called degenerative joint disease, is the most prevalent joint disorder in humans and animals (Romich, J. A. (1994) Top. Vet. Med. 5:16-23; Brooks, P. (2003) Bull. World Health Org. 81:689-690). As many as 20% of adult dogs are affected with OA, and suffer pain and disability as a result (Roush et al. (2002) Vet. Med. 97:108-112). OA can be defined as a disorder of movable joints, with associated deterioration of articular cartilage; osteophyte formation and bone remodeling; and changes in periarticular tissues. Although the condition is classified as a noninflammatory arthropathy, a low-grade, nonpurulent inflammation is common and several inflammatory components have been strongly associated with OA (Johnston et al. (1997) Vet. Clin. N. Am. Sm. Anim. Pract. 27:699-723; Amin et al. (1997) J. Clin. Invest. 99:1231-1237; Brooks et al. (2003) Bull. World Health Org. 81:689-690; Haynes et al. (2002) Clin. Immunol. 105:315-325). On a cellular and biochemical level, OA is associated with increases in degradative enzymes (especially the matrix metalloproteinases) released from chondrocytes in response to inflammatory cytokines. Inflammatory cytokines, such as interleukin-1β (IL-1), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNFα), as well as other inflammatory mediators, are increased in the synovial fluid of patients with OA.
Matrix metalloproteinases (MMPs), which are variably produced by chondrocytes, leukocytes and fibroblasts, include collagenases, stromelysins, gelatinases, elastase and others. All these enzymes break down cartilage matrix in some manner, and play an important role in physiologic remodeling of cartilage and other connective tissues. In OA, MMPs degrade glycosaminoglycans, including matrix glycoproteins, and collagen. They also reduce hyaluronic acid concentrations in the synovial fluid, leading to less viscous synovial fluid and impairing joint lubrication. Under normal conditions, the degradative processes of MMPs are appropriately balanced through the inhibitory function of tissue inhibitors of metalloproteinases (TIMPs). However, in OA, this balance is disrupted, with a disproportionate increase in MMPs. In addition, inflammatory cytokines, especially IL-1 and TNFα, stimulate the activation and release of MMPs.
Numerous studies, in dogs and other species, have documented increases in active MMPs, reductions in TIMP, or both, in OA. For example, it has been demonstrated that the degree of cartilage degradation in knee OA, as determined by arthroscopy, was strongly related to the activities of MMP-2 and MMP-13, as well as to the reduced inhibitory effect of TIMP-2 on MMP-2. Synovial fluid from dogs with naturally occurring OA has been shown to have higher MMP-2 activity, and dramatic increases in MMP-9 activity, compared to healthy controls (Volk S. W. et al. (2003) Am. J. Vet. Res. 64(10):1225-1233). MMP-9 has been correlated with rapidly destructive OA in the hip joint of women undergoing total hip replacement. Similarly, MMP-3 and MMP-9 were shown to be increased in blood, and MMP-1, MMP-3, MMP-9 and TIMP-1 all were shown to be increased in tissue samples from patients with this severe form of OA.
Given their important role in OA, it has been suggested that MMPs could serve not only as a therapeutic target for agents aimed at ameliorating cartilage destruction, but also may serve as useful markers for diagnosing and monitoring the progression of OA. An increase in MMP activity is stimulated by prostaglandins, including prostaglandin E2 (PGE2), which may be inhibited by non-steroidal anti-inflammatory drugs or other compounds that decrease PGE2 production.
The cytokines believed to be of greatest importance in OA include IL-1, IL-6 and TNFα. The cytokines and other inflammatory mediators in OA come from macrophages, lymphocytes, fibroblasts, synoviocytes and chondrocytes. Elevated concentrations of IL-1 and TNFα cause synovial inflammation as well as degradation of cartilage and proteoglycans through activation of MMPs. IL-1 stimulates the release of PGE2 from fibroblasts, which subsequently stimulate pain receptors. In addition, these cytokines stimulate the production of inflammatory free radicals, especially nitric oxide (NO).
The activity of IL-6 in synovial fluid is greatly increased in both dogs and humans suffering from OA. IL-6 can promote anabolic activity in OA through inhibition of MMP activation and promotion of matrix synthesis. On the other hand, IL-6 can stimulate MMP-2, MMP-9 and MMP-13. Thus, this pleiotropic cytokine helps reduce proteoglycan loss in the acute phase of OA, but enhances osteophyte formation in chronic phases. Several studies using IL-6−/− knock-out mice models have shown that IL-6 is critical to the development of arthritic lesions.
Other inflammatory agents involved in the pathogenesis of OA include the eicosanoids PGE2, thromboxane A2 (TXA2) and leukotriene B4 (LTB4), produced from arachidonic acid via cyclooxygenase-2 (COX-2) or 5-lipooxygenase (LOX) enzymes. The activity of these enzymes, and resulting eicosanoids, are increased in OA: osteoarthritic cartilage spontaneously releases 50 times more PGE2 compared to normal cartilage. LTB4 promotes the synthesis and release of IL-1 and TNFα. Further, LTB4 is a potent chemotactic agent and can increase neutrophil-induced damage to local tissues. TXA2 stimulates monocytes to release TNFα and IL-1, which subsequently promote MMP production and joint destruction. PGE2 promotes local inflammation and pain. It can promote osteoclastic bone resorption, increased destruction of Type II collagen and loss of proteoglycans. PGE2 stimulates IL-6 release from fibroblasts, and it also sensitizes chondrocytes to the effects of the free radical NO. Inhibition of the COX-2 enzyme results in a decrease in PGE2, as well as a reduction in IL-6.
There is no known cure for OA, so treatment is focused on controlling pain, improving joint function and slowing the degenerative process within the joint. Therapy usually involves weight management, controlled exercise, and anti-inflammatory and analgesic medications. It may also include nutritional supplements to help reduce inflammatory mediators, promote chondrocyte health and repair, and reduce oxidative damage.
Inhibition of the COX-2 enzyme responsible for PGE2 production is one means of providing relief for OA patients. Another means of reducing PGE2 production is through the use of dietary long chain omega-3 (n-3) polyunsaturated fatty acids (PUFA), which compete with arachidonic acid as substrates for the COX and LOX enzymes. Dietary long chain n-3 PUFA also suppress the pro-inflammatory mediators IL-1, IL-2 and TNF in cartilage tissue (Curtis, C. L. et al. (2000) J. Biol. Chem. 275(2):721-724).
Polyunsaturated fatty acids in both the n-6 and n-3 families can have immunomodulatory effects. The primary n-6 fatty acid in canine cell membranes is arachidonic acid (AA; 20:4n-6), which serves as the precursor for the production of PGE2, TXA2 and LTB4, potent inflammatory mediators in OA.
Polyunsaturated fatty acids of the omega-3 (n-3) or omega-6 (n-6 type) are not synthesized de novo in animal tissue and are required for normal cellular function. Thus, they are considered essential. The essential polyunsaturated fatty acids are linoleic acid (LA: 18:2n-6) and α-linolenic acid (ALA; 18:3n-3). When an animal is fed with a source of n-3 or n-6 polyunsaturated fatty acids, including 18:2n-6, 18:3n-3, 20:5n-3, 22:5n-3, and 22:6n-6, there is a corresponding enrichment of n-3 and n-6 highly unsaturated fatty acids (HUFAs), specifically 20:4n-6, 20:5n-3, 22:5n-3, 22:6n-3, into the circulation and in tissue enrichment. Because the precursors of the n-3 and n-6 HUFAs can only be obtained from dietary sources, their relative abundance in tissues is limited by the availability of these precursors in the diet.
If the diet is enriched with long chain n-3 PUFA, specifically eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3), part of the AA in cell membranes will be replaced by these long chain n-3 fatty acids. EPA can serve as alternate substrate for the COX-2 and 5-LOX enzymes, resulting in a different and less inflammatory set of compounds, e.g., PGE3, TXA3 and LTB5 instead of PGE2, TXA2 and LTB4.
The majority of clinical studies evaluating long chain n-3 PUFA in arthritis have been in human patients with rheumatoid arthritis. Most of those studies showed positive benefits from long-chain n-3 PUFA supplementation. Patients were able to reduce or discontinue the use of non-steroidal anti-inflammatory drugs (NSAIDs) without experiencing pain or joint stiffness. The beneficial response appeared to be directly linked to the dosage and duration of time receiving the long chain n-3 PUFA supplements. Similar effects have been shown in dogs with OA. Twenty-two dogs with OA of the hip were given a fatty acid supplement marketed for dogs with inflammatory skin conditions (DVM Derm Caps, DVM Pharmaceuticals, Miami, Fla.) (Miller et al. (1992) Canine Pract. 17:6-8). When dosed according to the manufacturer's recommendation, 13 of 22 dogs showed noticeable improvement in their arthritic symptoms within two weeks (Miller et al., 1992, supra).
Glucosamine, an amino-sugar, is the principal component of the O-linked and N-linked glycosaminoglycans (GAGs) that form the matrix in connective tissues. Hyaluronan and keratan sulfate are composed, in part, of repeating units of acetyl glucosamine. A decrease in glucosamine synthesis by chondrocytes has been implicated in the decline in matrix GAGs found in OA. Oral supplementation with glucosamine in the management of OA has been evaluated. Essentially all trials evaluating glucosamine have been done with a purified salt, such as glucosamine sulfate or glucosamine hydrochloride. The applicability of these data to glucosamine from natural sources (animal or poultry cartilage) has not been described.
More than 50% of orally administered glucosamine is non-ionized at the physiologic pH of the small intestine and, as a small molecule, is readily absorbed. Most orally administered glucosamine is oxidized, with 70% of the associated radiolabel detected in exhaled CO2. However, approximately 10% is retained in tissue. Glucosamine has a stimulatory effect on chondrocytes, and is incorporated into the proteoglycans and collagen of extracellular matrix.
Several short and long-term, double-blinded, randomized trials evaluating glucosamine supplementation in human patients with OA of the knee were recently reviewed via meta-analysis. These studies documented significant improvement in clinical signs of OA with 1500 mg glucosamine per day. Two studies followed patients for three years and documented that oral glucosamine efficiently inhibited the long-term progression of OA. Similar studies on glucosamine alone in dogs are lacking. However, several in vitro and in vivo canine studies showed a benefit to a combination of glucosamine and chondroitin sulfate.
Oxidative stress plays an important role in both inflammation and tissue destruction in arthritis. Arthritic patients have reduced concentrations of serum vitamins A, E and C and other antioxidants, as well as increased markers of oxidative damage. These anomalies could be reversed with antioxidant supplementation. Several studies support the benefit of supplemental antioxidants for controlling the oxidative damage in OA.
In addition to nutrient modifications that may help address changes associated with OA directly, dogs need appropriately balanced nutrition to support normal maintenance and regeneration. Dietary deficiencies have been reported for antioxidant nutrients, B-vitamins, zinc, calcium, magnesium and selenium. Each of these nutrients plays a role in the normal maintenance of cartilage and other tissues. Therefore, it is important that dogs with OA receive diets that provide complete and balanced nutrition.
In addition to providing a source of amino acids for proteoglycan and collagen synthesis, dietary proteins are important for their role in helping to maintain an optimum body condition. Protein has several physiologic effects that may be beneficial for weight control: protein stimulates metabolism and protein turnover, induces thermogenesis and promotes satiety. During weight loss and subsequent weight maintenance, increased protein intake promotes loss of body fat with retention of lean body mass. These features of protein may be beneficial to help address excess body weight in dogs with OA.
Standard medical care for arthritic dogs includes weight management, controlled exercise, and anti-inflammatory and analgesic medications. There is a need in the art for additional methods of therapy for canines and other animals with osteoarthritis, as well as therapies for humans to reduce the effects of osteoarthritis.